The fabrication of semiconductor integrated circuits normally involves a sequence of stages. Many of these stages involve coating a semiconductor wafer with photoresist and exposing the coated wafer to light passing through a mask (also known as a reticle). The exposure of a wafer in this way is carried out using an exposure tool, normally part of a lithography bay, and thus the mask may be referred to as a lithography mask.
The same exposure tool may be used with different masks for different stages in a production sequence, or an exposure tool may be dedicated to a particular stage in the production sequence, and only use a single mask. In either case, the mask is normally removable from the exposure tool.
In factories where integrated circuits are manufactured, and in particular in a factory processes 300 mm wafers, there will normally be many exposure tools running at the same time. One mask may have to be used on more than one exposure tool, and a mask may therefore have to be transported from one exposure tool to another.
Masks are normally carried by a human operator between different exposure tools or between an exposure tool and a storage area. When being carried or stored, a mask will be kept in a mask container (also known as a mask holder) in order to protect it from damage and airborne particles. Usually, the containers are made from plastic.
Because in a typical integrated circuit factory a large number of masks are in circulation, co-ordinating the use of these masks to reduce the amount of time that an exposure tool has to wait for the necessary mask to arrive can be difficult.
It is an object of the invention to address these issues.